In the field of digital radiography different methods are already known.
One such method is based upon a conventional phosphor of which the prompt-emitting light is measured directly during the X-ray irradiation using a CCD sensor. The light emitted by the fluorescent phosphor is hereby optically conducted to a CCD. One such system is described in U.S. Pat. No. 5,519,751 where a medical x-ray image processor receives the input data from a two dimensional CCD sensor which is coupled to a phosphor screen by a light conducting member. Due to the fact that a prompt emitting phosphor is used as an x-ray radiation image converting medium, detection can only be performed simultaneous with the exposure. Because the detecting area of the CCD is relatively small (15 by 15 mm), only small X-ray images can be taken at the same time. The application is therefore restricted to e.g. dental exposures. It is only possible to read-out a larger image with a CCD using a lens to image a reduced image of the emitted light onto the CCD. In this case the original image is highly reduced which results in a very low collection efficiency. Then a good signal to noise ratio can only be reached using large X-ray doses. Furthermore, the detection of the radiation image using a prompt emitting phosphor has to be performed in situ. It is not possible to read the image at another location than at the location where it is recorded and at another time than at the time at which the X-ray exposure is performed. Therefore every X-ray apparatus needs to contain a CCD and imaging optics. Due to all these factors the realisation of an x-ray system using CCDs is still very costly and impractical. An other system described in U.S. Pat. No. 4,933,558 uses a stored energy releasing phosphor plate for image detection. After being exposed to an X-ray image the phosphor is stimulated with an appropriate light source and the image recorded in the phosphor plate is readout using a CCD imager. The CCD imager is coupled to the phosphor plate using a lens or Fibre optic bundle. Although this allows the use of a phosphor plate having a size of about 7 times the size of the CCD when using tapered fibre optics, this method is restricted due to the limited size of the phosphor plate which can be used when a certain image pixel resolution is desired. In another embodiment described in U.S. Pat. No. 4,933,558 the phosphor is readout using an array of linear CCDs (each having a certain length but which are only one pixel size wide) continuously scanning the whole phosphor plate with a constant speed. With this arrangement only a small amount of pixels is readout at the same time. It is impossible to obtain a thorough readout of the fluorescent image within a acceptable time period. In U.S. Pat. No. 4,953,038 is described an other system using a stimulable phosphor plate. The sensor used for readout is a CCD sensor operated in time delay integration mode. Although a longer readout time is obtained for each pixel, the method needs a continuous scanning movement of the readout sensor so a great care has to be taken to synchronize sensor movement and readout electronics. The number of readout pixels is relatively small so readout takes a long time.